leaving out Rmarkdown, I tried to open their model in R and generate the PDF from latex, even in that case errors are generated (using overleaf.com instead works correctly). I think there is something in the file that R doesn't like, I'll do it manually with Overleaf at this point...
Overleaf documentation provides great resources for beginners: 4.8 How to createillustrations? (1)Sometimes you can copy illustrations from the original paper (don’t forget to cite), but it will most frequently be better to modify them / create your own illustrations (2) For editing illustrations...